1. Technical Field
The present invention relates to an end-point detector and a method for quickly and accurately measuring a change in thickness in semiconductor and other wafers during chemical-mechanical polishing (CMP) of the wafer.
2. Background Information
The prior art discloses methods and apparatus for removing material by chemical-mechanical polishing (CMP) from the surface of a wafer in the production of ultra-high density integrated circuits. In a typical process, a wafer is pressed against a polishing pad in the presence of a slurry under controlled chemical, pressure, velocity, and temperature conditions. The slurry solution generally contains small, abrasive particles that abrade the surface of the wafer and chemicals that etch and/or oxidize the surface of the wafer. The polishing pad is generally a planar pad made from a porous material such as blown polyurethane. Thus, when the pad and/or the wafer moves with respect to the other, material is removed from the surface of the wafer by the abrasive particles (mechanical removal) and by the chemicals in the slurry (chemical removal).
Microsensor technology is heavily dependent on precision wafer thickness control during fabrication. Poor thickness control has resulted in deviation in predicted system functionality. Inconsistent reproducibility of sensor functionality has also been partially attributed to the lack of precision control of wafer thickness. The existence and seriousness of the problem has remained a major area of research.
The prior art discloses various methods and apparatus for obtaining a wafer of a desired thickness at the completion of the process. U.S. Pat. No. 4,793,895 to Kaanta, et al., for example, discloses an apparatus and method for monitoring the conductivity of a semiconductor wafer during the course of a polishing process. A polishing pad that contacts the wafer has an active electrode and at least one passive electrode, both of which are embedded in the polishing pad. A detecting device is connected to the active and passive electrodes as the wafer is lapped by the polishing pad. The etch end-point of the wafer is determined as a function of the magnitude of the current flow.
U.S. Pat. No. 5,242,524 to Leach, et al. discloses an apparatus for remotely detecting impedance adapted for use on a polishing machine wherein the end point of polishing for removing a surface layer during the processing of semiconductor substrates is detected. A first stationary coil having a high permeability core is wound having an air gap and an AC voltage is applied to the stationary coil to provide a magnetic flux in the air gap. A second coil is mounted for rotation on the polishing table, in a position to periodically pass through the air gap of the stationary coil as the table rotates. The second coil is connected at its opposite ends to contacts which are embedded in the surface of the polishing wheel. The contacts are positioned to engage the surface of the substrate being polished and provide a load on the second rotating coil when it is in the air gap of the stationary coil, will perturb the flux field therein as a function of the resistance of the load caused by the contacts contacting either a conducting surface or a non-conducting surface. This perturbance of the flux field is measured as a change in the induced voltage in the stationary cell which is converted to a signal processed to indicate the end point of polishing, the end point being when a metallic layer has been removed to expose a dielectric layer or when a dielectric layer has been removed to expose a metallic layer.
U.S. Pat. No. 5,433,651 to Lustig, et al. discloses an in-situ chemical mechanical polishing (CMP) process monitor apparatus for monitoring a polishing process during polishing of a workpiece in a polishing machine, the polishing machine having a rotatable polishing table provided with a polishing slurry, is disclosed. The apparatus comprises a window embedded within the polishing table, whereby the window traverses a viewing path during polishing and further enables in-situ viewing of a polishing surface of the workpiece from an underside of the polishing table during polishing as the window traverses a detection region along the viewing path. A reflectance measurement means is coupled to the window on the underside of the polishing table for measuring a reflectance measurement means providing a reflectance signal representative of an in-situ reflectance, wherein a prescribed change in the in-situ reflectance corresponds to a prescribed condition of the polishing process.
U.S. Pat. No. 5,667,629 to Pan, et al. discloses an apparatus and method for determination of the end-point for chemical-mechanical polishing of a layer of dielectric material formed on an integrated circuit wafer. A first voltage is generated which is proportional to the current supplying electrical power to the electric motor driving the polishing mechanism. The current is proportional to the rate of removal of dielectric material by the polishing process. The integral over time of the first voltage, which is proportional to the amount of dielectric material and is a function of the age of the polishing pad. When the integral over time of the first voltage is less than the reference voltage the polishing continues. When the integral over time of the first voltage is equal to the reference voltage or becomes larger than the reference voltage the polishing is stopped.
U.S. Pat. No. 5,777,739 to Sandhu, et al. discloses an end-point detector and a method for quickly and accurately measuring the change in thickness of a wafer in chemical-mechanical polishing processes. The end-point detector has a reference platform, a measuring face, and a distance measuring device. The reference platform is positioned proximate to the wafer carrier, and the reference platform and measuring device are positioned apart form one another by a known, constant distance. The measuring face is fixedly positioned with respect to the wafer carrier at a location that allows the measuring device engage the measuring face when the wafer is positioned on the reference platform. Each time the measuring device engages the measuring face with respect to the measuring device. The displacement of the measuring face is proportional to the change in thickness of the wafer between measurements.
U.S. Pat. No. 6,015,754 to Mase, et al. discloses an apparatus used to subject a target surface of a semiconductor wafer to a polishing treatment, by moving the target surface and a polishing surface of a polishing cloth relative to each other while supplying a polishing cloth relative to each other while supplying a polishing liquid between the target surface and the polishing surface. Electric resistance is measured between pairs of measuring points arranged on opposite sides of dicing lines on the target surface, while subjecting the target surface to the polishing treatment. The polishing treatment is caused to be ended by comparing detected values of a changing rate in measured values of the electric property with a reference value set to correspond to an end point of the polishing treatment.
Such prior art of thickness control during lapping and polishing involves intermittent stopping of the lapping process to measure the thickness by various means. Other methods include capacitive sensing, slurring monitoring, and optical measurement. In the case of reactive ion etching, end-point detection using optical emission spectroscopy has been employed as a method.
The main disadvantages of the prior art include the time wasted during intermittent lapping and stopping, lack precision of measurement, involve considerable measurement complexity and have attendant high costs.
It is an objective of this invention to provide a precision thickness capability during lapping and polishing of semiconductor wafers and other wafers such as dielectric wafers.
It is a further objective of the present invention to provide automatic control features in the lapping and polishing of semiconductor wafers and other such wafers.
It is a further objective of the present invention to provide a means for end-point detection during chemical etching in the production of semiconductor wafers and other such wafers.
It is a further objective of the present invention to provide a method and apparatus for producing very thin diaphragms or other such structures.
It is a further objective of the present invention to provide a simple and low cost means for producing semiconductor wafers and other such wafers, and in particular, to achieve a desired thickness in such wafers.
It is a further objective of the present invention to improve the ease of operation of equipment used in the manufacture of semiconductor wafers and other such wafers.
It is a still further objective of the present invention to provide a means for using multiple fuses across the wafer so as to capture distributed non-uniformities.
The method and apparatus of the present invention makes use of a fused etch-stop as a basis of lapping and polishing of wafers to predetermined thickness and improved precision. It will make it possible to lap and polish wafers down to thickness lesser than currently achieved, with minimal error, which will be governed by the thickness uniformity of the lapped wafer. As a result, it becomes possible to fabricate unique devices on the thin-film wafer, which is usually not practical. It will also make it possible to precisely detect the depth of a desired trench depth.
Recent advances in the process technology of semiconductors of dissimilar materials have made it possible to utilize the benefits of chemical-mechanical polishing (CMP), wafer bonding, and reactive ion etching to leverage development of novel devices based on heterogeneous integration technology. Thickness control of wafers during lapping and chemical-mechanical polishing and end-point detection during reactive ion etching have remained a major barrier toward harnessing the optimum benefit of heterogeneous integration. Uniform diaphragm thickness, for example, is highly desired in the fabrication of membranes used for pressure sensors. Uniform thickness across the wafer is also desired during the fabrication of special semiconductor materials such as silicon carbide where some special processes are required in order to achieve the desired functionality. End-point detection during deep reactive ion etching has remained a challenge. Polarity-dependent etches require intentional doping to facilitate effective etch-stopping at the junction. However, this method is process intensive and lacks the flexibility of choice of desired thickness.
It has been found to be possible to lap and polish wafers down to thickness less than 5 microns, with very small error percent, which will only be governed by the thickness uniformity of the wafer. As a result, it becomes possible to fabricate unique devices on thin-film wafers, which has not been practical. It will also make it possible to precisely detect the desired depth during reactive ion etching.
The mode of operation of the method of the present invention involves the utilization of two wafers, target wafer and carrier wafer. The target wafer is lapped and polished while the carrier wafer serves as platform on which the target wafer is attached. By the use of photolithography, holes are strategically etched on the back side of the target wafer. The depth of the holes corresponds to the desired final thickness of the thin-film wafer. Thin strips of metal are deposited and patterned to run end to end of the target wafer. In doing so the strips would run directly over the holes maintaining a step coverage. The base width of the dimple can be made very small which may be less than the width of the eventual grids that define the boundary of a die on the wafer. A second wafer, the carrier, is obtained and an electrically insulating material (or the wafer may be insulating) is deposited on one of its face. The face of the first wafer with the holes and conductor strip is brought to intimate contact with the face of the carrier wafer with the bonding material. The diameter of the carrier wafer can be intentionally made slightly smaller than the first wafer so that when the wafers are mated and bonded, the first wafer will extend further out to reveal the conductor strip at the edge of the first wafer. Electrical contact can then be made at the end terminals of the conductor strip. Alternatively, through-holes are etched in the carrier wafer such that portions of the conducting strip are exposed to facilitate external electrical contact.
In the functional operation of the method of this invention, the entire unit is then mounted on the lapping machine with the un-etched face of the target wafer facing down. Connecting wires are run from the strip contact to an oh meter that monitors the resistance between any two terminals of the strip or a multiple of them. The conductor can be made to become part of the electrical system of the equipment so that it functions as a variable resistor. When the lapping process reaches the conductor, the conductor will also be lapped and the resistance will start increasing. When the lapping process eventually breaks the conductor, an open circuit is observed, which signifies that the desired thickness has been achieved and the xe2x80x9cfusexe2x80x9d will open. This opening will cause the flapping equipment to stop automatically, thus obtaining the precise thickness. In the case whereby the slurry or polishing wheel is conducting, complete fusing may not be achieved, but the abrupt change in resistivity can be utilized as a control mechanism.